The present invention relates to methods for removing nitrogen oxides effectively from exhaust gas containing the nitrogen oxides such as exhaust gas from internal combustion engines of automobiles etc, exhaust gas from consumers appliance such as cooking apparatus and the like, and exhaust gas from boilers in factories and thermal power stations.
Further, the present invention relates to catalysts for removing nitrogen oxides from the exhaust gas. The catalysts relating to the present invention are preferable for purifying the exhaust gas released from lean burn engines.
Nitrogen oxides (NOx) are contained in exhaust gas released from internal combustion engines of automobiles etc. The nitrogen oxides are harmful to human bodies, and become an origin to destroy a global environment by acid rain etc. Therefore, various catalyst s for removing the nitrogen oxides in exhaust gas have been investigated.
Most of catalysts for purifying exhaust gas from automobiles which are under developing at present have been aimed at treating exhaust gas from automobiles wherein a ratio of fuel to air, that is air/fuel by weight, is set at approximately stoichiometric ratio, that is theoretical air to fuel ratio (air/fuel=14.7 by weight). Combustion under the stoichiometric condition generates and releases hydrocarbons and carbon monoxide in addition to the nitrogen oxides. The hydrocarbons and carbon monoxide are also environment contaminating material. Therefore, three way catalyst which can treat the above three substances simultaneously have been a main object of the development. The three way catalyst is a general name for catalysts which can simultaneously treat nitrogen oxides, hydrocarbons, and carbon monoxide in exhaust gas. Most of the three way catalysts contain noble metals such as rhodium, palladium, and platinum as for main components.
However, currently, lean burn engines which burn fuel under a fuel to air ratio (air/fuel) larger than the theoretical fuel to air ratio are becoming a main current as for automobile engines in view of decreasing fuel consumption. With the lean burning, oxygen content in the exhaust gas increases, and an activity of conventional three way catalyst decreases in the presence of the oxygen. Accordingly, nitrogen oxides in the exhaust gas can not be removed effectively. Therefore, development of catalyst for purifying exhaust gas from lean burn engines becomes necessary.
As the catalyst for purifying exhaust gas from lean burn engines, a catalyst which is composed of copper supported by zeolite (JP-A-1-130735 (1989), Proceeding of 68th Meeting for discussing catalyst, 3F108, (1991)), and a catalyst which is composed of cobalt and rare earth metals supported by zeolite and further at least one of copper and rhodium is supported by the zeolite (JP-A-4-219147 (1992)) are disclosed.
A catalyst which absorbs NOx under a lean burning condition, desorbs the absorbed NOx under a stoichiometric burning condition, and reduces the NOx is disclosed in JP-A-5-261287 (1993). The catalyst is composed of barium oxides, lanthanum oxides, and platinum, all of which are supported by an alumina supporter.
All the above described conventional catalysts for purifying exhaust gas from lean burn engines have such a problem that the catalysts lack a long durability, because all the catalysts contain zeolite. Under the lean burning condition, water is generated approximately 10% by volume by combining hydrocarbons and oxygen in the exhaust gas. Zeolite has such a property that the zeolite loses zeolite structure when it is heated under a condition existing water. Once the zeolite structure is broken, active components supported by the zeolite coagulates, and the catalytic activity decreases.
The catalyst which is composed of barium oxides, lanthanum oxides, and platinum, all of which are supported by an alumina supporter has such a problem that the barium oxides which is contained by a high concentration are thermally deteriorated.